An electrophotographic, or xerographic, image printing system employs an image bearing surface, such as a photoreceptor drum or belt, which is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the image bearing surface is exposed to a light image of an original document being reproduced. Exposure of the charged image bearing surface selectively discharges the charge thereon in the irradiated areas to record an electrostatic latent image on the image bearing surface corresponding to the image contained within the original document. The location of the electrical charge forming the latent image is usually optically controlled. More specifically, in a digital xerographic system, the formation of the latent image is controlled by a raster output scanning device, usually a laser or LED source.
After the electrostatic latent image is recorded on the image bearing surface, the latent image is developed by bringing a developer material into contact therewith. Generally, the electrostatic latent image is developed with dry developer material comprising carrier granules having toner particles adhering triboelectrically thereto. However, a liquid developer material may be used as well. The toner particles are attracted to the latent image, forming a visible powder image on the image bearing surface. After the electrostatic latent image is developed with the toner particles, the toner powder image is transferred to a media, such as sheets, paper or other substrate sheets, using pressure and heat to fuse the toner image to the media to form a print.
The image printing system generally has two important dimensions: a process (or a slow scan) direction and a cross-process (or a fast scan) direction. The direction in which an image bearing surface moves is referred to as the process (or the slow scan) direction, and the direction perpendicular to the process (or the slow scan) direction is referred to as the cross-process (or the fast scan) direction.
Electrophotographic image printing systems of this type may produce color prints using a plurality of stations. Each station has a charging device for charging the image bearing surface, an exposing device for selectively illuminating the charged portions of the image bearings surface to record an electrostatic latent image thereon, and a developer unit for developing the electrostatic latent image with toner particles. Each developer unit deposits different color toner particles on the respective electrostatic latent image. The images are developed, at least partially in superimposed registration with one another, to form a multi-color toner powder image. The resultant multi-color powder image is subsequently transferred to a media. The transferred multicolor image is then permanently fused to the media forming the color print.
Banding generally refers to periodic defects on an image caused by a one-dimensional density variation in the process (slow scan) direction. Various sources of banding exist in a image printing system and the frequencies of these sources are typically known from a mechanical design of the image printing system. Bands can result due to many different types of variations within components and/or subsystems, such as roll run out (variations in roll or drum diameter) in a developer roll or photoreceptor drum, wobble in the polygon mirror of the laser raster optical scanner (ROS), and the like.
Banding compensation may be performed by modulating the power or intensity of one or more exposing devices, where exposure power is modulated for each scan line based on the banding profile. For examples of methods and systems for modulating exposure power or intensity, see, e.g., U.S. Pat. Nos. 7,492,381, 6,359,641, 5,818,507, 5,659,414, 5,251,058, 5,165,074 and 4,400,740 and U.S. Patent Application Pub. No. 2003/0063183, each of which herein incorporated by reference in its entirety.
To modulate the exposure power for banding compensation, a banding profile may be determined and analyzed. For example, see U.S. Patent Pub. Nos. 2009/0002724 and 2007/0236747, each of which herein incorporated by reference in its entirety, for examples of systems and methods for measuring the frequency, amplitude, and/or phase of bands. Banding profiles analyses may involve printing several pages of uniform halftone image, and measuring the prints using an offline or inline spectrophotometer, scanner, or density sensor. The image data is averaged in the cross process direction to obtain one-dimensional profiles in the process direction which are then analyzed for banding.
An exposure correction determined from the banding profile may be applied to a Tone Reproduction Curve (TRC). For examples of TRCs, see, e.g., U.S. Pat. Nos. 5,963,244 and 6,694,109, each of which herein incorporated by reference in its entirety. However, the resulting performance can be unsatisfactory as certain parts of the TRC may be undercompensated and certain parts overcompensated.